A power plant needs access to water for cooling. Belarus is a land-locked country without access to sea, so all of its rivers also flow through other countries. Its watershed is divided between Nemunas, Daugava, and Dnieper, which have Vilnius, Riga, and Kiev situated on them. It seems like there's no place you can build a power plant there without risking a potential contamination of water which would then run through a capital of another state.
Also, Lithuanian Ignalina power plant was using the water eventually getting into Daugava river, running through Riga, a capital of the neighboring Latvia. I can't see why Lithuania itself thinks it's ok to risk potential contamination of water of a neighboring country capital, but not ok when someone else is doing the same to them.